Photoflash lamp

ABSTRACT

A photoflash lamp in which the filamentary combustible within the lamp envelope comprises a plurality of fine strands having a length determined by a relationship involving the inside diameter of the envelope, the weight of the combustible, and a constant dependent upon the composition and cross-section of the strands. In subminiature lamps, high strand density provides support for strand lengths equal to or less than the inside diameter of the lamp envelopes. The combustible is distributed with a minimum strand density at the envelope walls and increasing toward the center of the lamp envelope.

United States Patent [1 1 Broadt et al.

[ July 22, 1975 PHOTOFLASH LAMP [75] lnventors: David R. Broadt, Lewisburg; Donald E. Armstrong, Williamsport, both of Pa.

[73] Assignee: GTE Sylvania Incorporated,

Montoursville, Pa.

[22] Filed: Sept. 9, 1971 [21] Appl. No.: 179,056

[52] U.S. Cl. 431/93; 431/95 [51] Int. Cl. F21K 5/02 [58] Field of Search 431/93, 94, 95

[56] References Cited UNITED STATES PATENTS 2,857,752 10/1958 Anderson et al. 431/94 2,982,119 5/1961 Anderson 431/94 Primary Examiner-Carroll B. Dority, Jr. Assistant Examiner-Larry l. Schwartz Attorney, Agent, or Firm-Edward J. Coleman [57] ABSTRACT A photoflash lamp in which the filamentary combustible within the lamp envelope comprises a plurality of fine strands having a length determined by a relationship involving the inside diameter of the envelope. the weight of the combustible, and a constant dependent upon the composition and cross-section of the strands. ln subminiature lamps, high strand density provides support for strand lengths equal to or less than the inside diameter of the lamp envelopes. The combustible is distributed with a minimum strand density at the envelope walls and increasing toward the center of the lamp envelope.

4 Claims, 3 Drawing Figures PATENTED JUL 2 2 I975 SHORT SHREDS O mZMEDJ 0m2 320M MILLISECONDS FIG.3

PHOTOFLASH LAMP BACKGROUND OF THE INVENTION This invention relates to photoflash lamps and particularly to subminiature and smaller photoflash lamps containing a filamentary combustible of the shredded foil type.

Subminiature photoflash lamp, i.e. lamps having an envelope volume of less than one cubic centimeter, are presently mass produced in large quantities for use in the small photographic flashlamp units referred to as flashcubes. As described in U.S. Pat. No. 3,244,087, flashlamp units of this type comprise: a container hav ing a plurality of closed transparent sides; a plurality or reflectors disposed in the container, one along each side thereof; and a photoflash lamp disposed in operative relationship with respect to each of the reflectors. Each of the subminiature flashlamps employed in the unit consist of a hermetically sealed, light transmitting glass envelope which contains a filamentary combustible material, such as shredded zirconium foil, and a combustion-supporting gas, such as oxygen. In lamps intended for battery operated flash systems, the envelope also includes an electrical ignition system comprising a tungsten filament supported on a pair of lead-in wires having a quantity of ignition paste on the inner ends thereof adjacent to the filament. This type of lamp is operated by the passage of electric current through the lead-in wires. In the case of percussive-type photoflash lamps, such as described in U.S. Pat. No. 3,535,063, a mechanical primer is sealed in one end of the lamp envelope. The primer may comprise a metal tube extending from the lamp envelope and a charge of fulminating material on a wire supported in the tube. Operation of the percussive photoflash lamp is initiated by an impact onto the tube to cause deflagration of the fulminating material up through the tube to ignite the combustible disposed in the lamp envelope. The percussive-type lamps are employed in multi-lamp cubical units having respective pre-energized striker springs for each lamp as described in U.S. Pat. No. 3,597,604.

Such small size flash lamps and the compact, disposable, multi-lamp units in which they are employed have gained wide popularity with camera users because of their advantages of greater convenience, compactness and portability. However, it has been observed in some cases that as the size of the flash lamp envelope is reduced, the efficiency of burning of the combustible may be adversely affected. One hypothesis for this decrease in efficiency holds that the distance for the burning particles of combustible material to travel through before striking the envelope wall is too short to permit the complete combustion of all the burning particles before so striking the glass wall. As a result, an appreciable percentage of the burning particles become cooled to such an extent by their contact with the relatively cool envelope wall as to prevent their complete combustion and therefore the fullest utilization of their light producing potential.

As briefly mentioned above, the combustible mate rial commonly employed in presently available photoflash lamps consists ofa loosely distributed quantity of filamentary material of a type commercially known as shredded foil. The material is made by cutting or shredding a thin sheet of ribbon of suitable metal foil into thin strands. Aluminum and magnesium foil have been used for this purpose, although more recently, the use of zirconium has been found to provide significant photometric advantages, as described in U.S. Pat. No. 3,067,601. Originally, it was customary to employ shredded foil strands having a length of the order 8 inches. The use of such long strands in small size lamps, however, increased the difficulty of obtaining good distribution of the shredded foil by the pneumatic foilloading methods which are conventionally used. Poor distribution of the shredded foil in the lamp envelope adversely affects the timing of the light flash and also decreases the efficiency of burning of the combustible material, thereby resulting in reduced light output from the lamp. To improve the distribution and ignition characteristics of the shredded combustible in smaller size lamps, U.S. Pat. No. 2,857,752, Anderson et al, proposes the use of finely cut strands of metal foil having a cross-sectional area in the order of 1 mil and a strand length within the range of l to 5 inches. According to the patent, the use of these shorter strands of shredded foil has the advantage, of affording a great many more loose ends serving as ignition points for a given weight or charge of shredded foil in the lamp, thereby facilitating the ignitionof the shredded foil in a manner which tends to produce increased useful light output therefrom. It is observed that if the strands of shredded foil are too short, they will not maintain themselves in place in the lamp envelope in distributed position, but instead will drop down or collapse into the bottom region of the lamp. To prevent such collapsing of the shredded foil in the bulb and to assure its proper self support, the Anderson patent teaches that it is necessary to maintain the individual foil strands above a minimum length of the order of 1 inch or a length such that the ratio of the individual strand length in inches to the maximum envelope diameter in inches is at least approximately 1.3.

In addition to the shorter strand length, the Anderson patent instructs that the filamentary material should be distributed loosely within the bulb and as uniformly as possible throughout the space occupied thereby in order to insure the most favorable combustion conditions for the material.

Although providing an improved light output, sub miniature flash lamps produced according to the shortened strand specifications of the Anderson patent were unable to provide the light output requirements for certain photographic applications. In order to achieve the desired higher levels of light output from flash lamps having an envelope volume of less than 1 cubic centimeter, shredded hafnium foil was employed as the filamentary combustible. Although providing a very significant improvement in light output, the use of hafnium as the combustible results in a relatively expensive flash lamp unit. The exotic metal hafnium is itself much more expensive than zirconium or aluminum. In addition, however, the combustion of hafnium is more violent than that of zirconium or aluminum, thereby requiring a hard glass envelope, such as the borosilicate glass envelope described in U.S. Pat. No. 3,506,385, to provide adequate containment. This incurs added expense, which is further increased by the special lead-in wires required for compatibility with the hard glass.

SUMMARY OF THE INVENTION In view of the foregoing, one of the principal objects of this invention is to provide a photoflash lamp which provides an improved efficiency of combustion, thus improved light output, using the more common combustible materials, such as zirconium, without having to resort to expensive and exotic metals such as, hafnium, scandium, indium, alloys, or laminates.

A particular object of the invention is to improve the photometric output of subminiature and smaller photoflash lamps by improving the efficiency of the burning shredded combustible.

Another object is to provide a high light output photoflash lamp which may be more economically produced than like-sized lamps heretofore available for providing similar light output characteristics.

These and other objects, advantages and features are attained, in accordance with the principles of this invention, pursuant to our discovery that the strands of shredded combustible may be shortened to a length even less than the internal diameter of the lamp to provide an increased efficiency of combustion without loss of supportability by recognizing that optimum shred length can be determined by a relationship involving the inside diameter of the lamp, the effective internal bulb length, the quantity, or weight, of the combustible, and the composition and cross-section of the strands. The photoflash lamp in accordance with the invention employs a filamentary combustible having a strand length inversely related to the weight of the filamentary material, with the filamentary material being made selfsupporting within the lamp envelope by appropriately increasing the density of the strands.

We have further discovered that, contrary to previous teachings relative to the desirability of a loose, uniform distribution of the filamentary material, the use of combustible strand lengths slightly longer than or less than the internal diameter of the envelope of subminiature photoflash lamps provides increased efficiency of combustion by virtue of a distribution of combustible material whereby strand density is minimal at the envelope walls and increases toward the center of the envelope.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational view of an electrically ignitable photoflash lamp containing a filamentary combustible in accordance with the invention;

FIG. 2 is a sectional elevation of a precussive-type photoflash lamp containing a filamentary combustible in accordance with'the invention; and

FIG. 3 is a plot of light output in terms of zonal megalumens against time in milliseconds for 4 inch shreds and short shreds in similar envelopes.

DESCRIPTION OF PREFERRED EMBODIMENT The teachings of the present invention are applicable to either percussive or electrically ignited photoflash lamps of a wide variety of sizes and shapes; however, the invention is particularly advantageous as applied to flash lamps having tubular shaped envelopes with a volume ofless than 1 cubic centimeter (cc.). Accordingly, FIGS. 1 and 2 respectively illustrate electrically ignited and percussive-type photoflash lamps embodying the principles of the invention.

Referring to FIG. 1, the electrically ignitable lamp comprises an hermetically sealed lamp envelope 2 of glass tubing having a press 4 defining one end thereof and an exhaust tip 6 defining the other end thereof. Supported by the press 4 is an ignition means comprising a pair of lead-in wires 8 and 10 extending through and sealed into the press. A filament l2 spans the inner ends of the lead-in wires, and beads of primer 14 and 16 are located on the inner ends of the lead-in wires 8 and 10 respectively at their junction with the filament; Typically, the lamp envelope 2 has an ID. of less than one-half inch, and an internal volume of less than 1 cc. A combustion-supporting gas, such as oxygen, and a filamentary combustible material 18, such as shredded zirconium foil, are disposed within the lamp envelope as will be described in more detail hereinafter.

The percussive-photoflash lamp illustrated in FIG. 2

comprises a length of glass tubing defining an hermetically sealed lamp envelope 22 constricted at one end to define an exhaust tip 24 and shaped to define a seal 26 about a primer 28 at the other end thereof. The primer 28 comprises a metal tube 30, a wire anvil 32 and charge of fulminating material 34. A combustible such as filamentary zirconium 36 and a combustionsupporting gas such as oxygen are disposed within the lamp envelope, as will be detailed hereinafter. The wire anvil 32 is centered within the tube 30 and is held in place by a circumferential indenture 38 of the tube 30 which laps over the head 40 or other suitable protuberance at the lower extremity of the wire anvil. Additional means, such as lobes 42 on wire anvil 32 for example, may also be used in stabilizing the wire anvil, supporting it substantially coaxial within the primer tube 30 and insuring clearance between the fulminating material 34 and the inside wall of the tube 30. A refractory bead 44 is fused to the wire anvil 32 just above the inner mouth of the primer tube 30 to eliminate burnthrough and function as a deflector to deflect and control the ejection of hot particles of fulminating material from the primer. The lamp of FIG. 2 is also typically a subminiature type having envelope dimensions similar to those described with respect to FIG. 1.

Although the lamp of FIG. 1 is electrically ignited, usually from a battery source, and the lamp of FIG. 2 is percussion-ignitable, the lamps are similar in that in each the ignition means is attached to one end of the lamp envelope and disposed in operative relationship with respect to the filamentary combustible material. More specifically, the igniter filament 12 of the flash lamp in FIG. 1 is incandesced electrically by current passing through the metal filament support leads 8- and 10, whereupon the incandesced filament ignites the beads of primer 14 and 16 which in turn ignite the combustible 18 disposed within the lamp envelope. Operation of the percussive-type lamp of FIG. 2 is initiated by an impact onto tube 30 to cause deflagration of the fulminating material 34 up through the tube to ignite the combustible 36 disposed within the lamp envelope.

As previously mentioned, the filamentary combustible employed in flash lamp envelopes normally comprises a shredded metallic foil, such as zirconium. The metallic foil manufactured for this application is normally provided in thicknesses of about or somewhat less than 1 mil and in widths of about 2 to 4 inches. The foil is then processed through standard shredding equipment to produce desirable cross sections of about 1.2 to 2.0 square mils, depending upon the characteristics of the various photoflash lamps. Accordingly, the strands of shredded foil are normally about 2 to 4 inches long and in some instances, as described by the aforementioned Anderson et al patent, the foil may be split in half to provide 1 inch strand lengths for the filamentary combustible.

As discussed hereinbefore, this relatively short strand combustible has been found useful for improving the light output in smaller sized flashlamps, with the prior art teaching that a minimum length of 1 inch or 1.3 times the bulb diameter was necessary to render the combustible self-supporting. Even when using these relatively short shreds of metallic foil, however, there still results an undesirably low efficiency of combustion in subminiature flashlamps. It will be noted that these strand lengths, although relatively short, are nevertheless more than two times greater than the ID. of a submarine lamp envelope, which is typically much less than one-half inch. When these long shreds are introduced into the lamp, they tend to lie along the wall of the lamp in a helical configuration, assuming the curvature of the wall, with a relatively small amount of fill in the center of the lamp. It is clear that this type of fill distribution will even hold true for strands having a minimum length of 1.3 times the ID. of the subminiature lamp.

We have discovered that by modifying the length of the shreds being used to fill the lamp, this tendency of the normally employed shreds to conform to the wall curvature can be greatly reduced, resulting in higher light output. In particular, testing has shown that by shortening the shred length so that it is slightly longer than or less than the lamp diameter, a significantly increased efficiency is seen to result. ln view of prior art teaching, however, it would at first appear that the use of such short shreds would result in a tendency of the combustible to settle in the lamp and provide a poor fill distribution, thereby partially negating the increased light output. Unexpectedly, however, we have discovered that a mass of filamentary combustible material having a strand length slightly longer than or less than the [.D. of the subminiature lamp envelope can be successfully rendered self-supporting by appropriate selection of the strand density, or more specifically the weight of the combustible. In addition, we have discovered that the use of such short strands causes the distribution of the combustible to be minimal at the envelope walls and more dense toward the center of the lamp, a feature which appears to particularly counteract the efficiency reducing effects attributable to the wall hugging characteristics of conventional longer shreds. More importantly, we have determined that an optimum strand length, with respect to maximizing the efficiency of combustion, is dependent upon a relationship involving the inside diameter of the lamp envelope, the effective internal bulb length, the weight of the combustible, and the composition and cross-section of the strands.

in accordance with the present invention, the strand length, in inches, of the filamentary combustible mate rial for photoflash lamps is determined by the following formula:

Strand Length K where [D is the inside diameter of the lamp envelope in inches, W is the weight of the filamentary material within the lamp envelope in milligrams, L is the average effective internal length of the lamp envelope in inches, and K is a constant dependent upon the composition and cross-section of the strands of filamentary material. The constant K is determined empirically, for example, a constant K 22.3 has been found suitable for shredded zirconium with a 1.1 to 1.2 square mil crosssection; and a constant K 36.6 appears to be suitable for shredded hafnium with a 1.1 to 1.2 mill crosssection.

The above formula tends to optimize the length of the combustible strands for the various lamp envelope sizes and till arrangements that may be desired. The constant K takes into consideration the fact that the composition of the combustible shreds may vary for different lamp designs dependent upon the cost considerations and the light output requirements of the applications for which that lamp is directed. As the crosssection of the shredded strands is significant factor in the determination of flash timing, K also takes this design factor into consideration. The strand cross-section also determines individual strand rigidity or supportability as well as the number of strands; hence it is desirable to optimize to the smallest usable cross-sections so as to obtain the greatest I number of individual strands. It is interesting to note here that short strands of the same cross-section as the regular length 2 inch 4 inch strands unexpectedly indicated a retardation of time to peak by 0.5 1.5 milliseconds, thus making it feasible to utilize smaller cross-sections than had heretofore been in use with zirconium foil. Prior art had indicated the use of larger cross-sections to obtain maximum total efficiency from the foils.

Of course, the formula also accounts for the variation in internal diameter of various lamp types. In this connection, it is interesting to note that the relationship indicates that strand length decreases faster than a reduction in the envelope internal diameter. Hence, in microminiature and suliminiature sized lamps, the strand length can be significantly shorter than the ID. of the lamp envelope. Experimentation has shown the formula to hold true for various flash lamp envelope sizes having internal diameters less than /2 inch, and although the formula may hold true for larger size lamps, no testing has been made of lamps over /2 inch.

With respect to the remaining relationship, it will be noted that weight is inversely related to strand length and thus strand density will increase for shorter strand lengths. Further, it has been observed that it is this fill weight, or strand density, factor which contributes to the self-supporting characteristics of the mass of filamentary combustible within the lamp. Thus, assuming a given lamp size and a predetermined fill composition and cross-section, strand length and fill weight are the remaining variables. If the strand length is shortened, the number of strands loaded into the lamp envelope must be increased to provide the necessary selfsupporting characteristics. To achieve a maximum efficiency of combustion, it is desirable to make the strand length as short as possible; hence, the primary limitation on strand length is the practical maximum that must be placed on fill weight. There is a certain point beyond which an increase in fill weight will pose containment problems in economically feasible envelope designs, and another point beyond which further increases in the density of the combustible fill will actually reduce the efficiency of combustion and thus the light output.

An additional contribution to increased burning efficiency and improved light output is provided by the combustible fill distribution assumed by the use of short strand lengths determined in accordance with the invention. Hereinbefore it has been discussed how the conventional long strand fill tended to distribute itself as swirling clusters near the lamp envelope wall and for that reason resulted in reductions in the efficiency of combustion. When using conventional foil-loading methods, the short shred combustible in accordance with the invention distributes itself within the lamp envelope with the strands tending to be most dense toward the center of the envelope and least dense at the walls thereof. Upon ignition and pursuant combustion of the fill, this distribution, with a greater mass of the fill away from the cool walls of the lamp envelope, results in a greater persistence of glowing, i.e. the burning particles of shredded foil are held for a longer period of time in the envelope space before being quenched by the cool walls thereof. This is shown in FIG. 3 by the straightness of the decay of the short shred curve A.

A further enhancement of photomeric output is obtained if the body of combustible material is concentrated in the upper two-thirds of the lamp envelope; i.e. the filamentary material occupies approximately twothirds of the envelope toward the end thereof opposite to that to which the ignition means is attached. This arrangement of the fill wherein it occupies the upper twothirds of the envelope and has a density gradient increasing toward the center of the envelope is illustrated by the distribution of the filamentary combustible materials l8 and 36 in FIGS. 1 and 2 respectively. Restriction of the combustible to the upper two-thirds of the envelope is provided by subjecting the glass envelope to a mechanical shock subsequent to the foil-filling process.

The following table illustrates the strand lengths and fill weight concentration of zirconium shredded foil determined in accordance with the invention for four different lamp sizes along with the substantial increase in light output obtained in each case. The strand length figures vary slightly from that obtained by use of the aforementioned formula as attempts were made to equally divide the available foil stock widths to avoid the creation of unusable scrap.

a strand length of 0.286 inch. The lamp was of the electrically ignitable type, and the ordinate axis in FIG. 3

is expressed as zonal megalumens as the lamp was measured as employed in a flashcube, the reflectors thereof 5 tending to zone the light. For comparison, curve B shows the light output characteristic for a similar envelope filled with conventional 4 inch shreds of zirconium.

A further advantage of the relationship for determining strand length in accordance with the invention is the greater concentration of fill weight which results, thereby further contributing to increased light output. Clearly, the fill concentration will never be less than approximately milligrams per cubic centimeter. For example, in the table above, the respective concentrations of the zirconium fill from left to right are 105 mg./cc., 77 mg./cc., 29.5 mg./cc., and 38.5 mg./cc.

The short shreds of the present invention also provide a significantly higher color temperature thereby enabling a reduction in density of the blue coating required for the photoflash lamp to provide a further improvement in photometric output. For example, the integrated correlated color temperature (0 msec) for the 0.270 O.D. lamp in the above table is experimentally determined to be approximately 4,872K, plus or minus 75K, as compared to a color temperature of 4,725K, plus or minus 25K, for a similar size lamp filled with 4 inch strands of zirconium.

Test data obtained from thousands of flashed lamps has also indicated a further unexpected advantage of using short shred lengths in accordance with the invention, namely, the combustion appears to be less violent thereby providing a much safer lamp having a substantially lower probability of explosive failure.

sufficient to render said filamentary material self- (control) (test) 25% Curve A of FIG. 3 illustrates the light output characteristic of the 0.400 inch O.D. lamp in the above table, i.e. having a fill of the short shreds of zirconium with supporting within said'envelope, and said filamentary material being distributed within said envelope with the strands thereof most dense toward the of said strands of filamentary combustible material is sufficient to substantially maintain the spatial distribution of said strands within said envelope independent of the orientation of said envelope.

4. A lamp according to claim 1 wherein said envelope is substantially tubular, and said strands of filamentary combustible material have a length less than about 1.2

times the internal diameter of said envelope. 

1. A photoflash lamp comprising: an hermetically sealed, light transmitting envelope; a combustion-supporting gas in said envelope; a quantity of filamentary combustible material located within said envelope, said filamentary material comprising a plurality of strands having a length inversely related to the weight of said filamentary material, the density of said strands being sufficient to render said filamentary material self-supporting within said envelope, and said filamentary material being distributed within said envelope with the strands thereof most dense toward the center of said envelope and least dense at the walls of said envelope; and ignition means attached to one end of said envelope and disposed in operative relationship with respect to said combustible material.
 2. A lamp acccording to claim 1 wherein said filamentary material occupies approximately two-thirds of said envelope toward the end thereof opposite that to which said ignition means is attached.
 3. A lamp according to claim 1 wherein the density of said strands of filamentary combustible material is sufficient to substantially maintain the spatial distribution of said strands within said envelope independent of the orientation of said envelope.
 4. A lamp according to claim 1 wherein said envelope is substantially tubular, and said strands of filamentary combustible material have a length less than about 1.2 times the internal diameter of said envelope. 